|
Quarterdeck
Volume 5, Number 1, Spring 1997
Mary Jo Richardson
Wilford D. Gardner
Conductivity-Temperature-Depth
meter (CTD)
The primary
workhorse for water column sampling is the conductivity-temperature-depth
meter (CTD), an electronic package that we deploy over the
side of the ship at sampling stations.
The package
measures conductivity by passing a current through the water.
We convert the conductivity measurement to salinity by comparing
the result to conductivities of water with known salinities.
The CTD measures temperature with an electronic thermometer
and depth with a pressure sensor.
Salinity,
temperature, and pressure are used to calculate water density.
These four parameters are the basic pieces of information
we always need to evaluate other data from seawater.
The electronic
information produced by most of the other underwater instruments
discussed in this article can be linked through the CTD. The
data travel to the ship through a conducting cable and we
view them in real time on a computer screen.
We can
also surround the CTD with a rack of water bottles that are
open at both ends. As the bottles descend their ends can be
closed at whatever depths we desire depending on the water
characteristics revealed by the other instruments. On the
surface we analyze the water and its particles for the property
we are studying.
|
CTD [33K]
Bottles arranged in a rosette formation on a rack with a CTD
can be closed at different depths to collect water for later
analysis in the laboratory.
|
Transmissometer
Transmissometers
measure the transmission of light of a given wavelength over
a known distance in seawater. Different types of particles
and dissolved organic matter in the water absorb and reflect
light of different wavelengths. We choose wavelengths for
our instruments based on which particles we are studying.
The wavelength we use the most is about 660 nanometers (red).
Surprisingly
and fortunately, the amount of dissolved salt in the water
does not affect the transmission of light through it. Transmission
through particle-free fresh water and salt water is the same.
Changes
in the transmission of light through water are primarily related
to changes in the abundance and type of particles present.
Most variations in transmission come from particles less than
20 microns in diameter. Large particles and aggregates larger
than 500 microns in diameter are not abundant in the ocean.
Only a few exist in 1000 milliliters of water, so they rarely
appear in the small sensing volume of the transmissometer
(45 milliliters). When they do, they appear as singular large
values which we remove from our data.
|
Transmissometer [18K] |
Attenuation
The
amount of attenuation of light (c) equals the sum of light
scattering (b) and absorption (a).
It's as simple as a+b=c.
Measuring
these values is not so simple.
Most
attenuation results from light scattering. Phytoplankton,
however, contain little packets of chlorophyll in their cells
that act as sponges for light, allowing them to absorb energy
and use it in photosynthesis. Thus, living phytoplankton absorb
more light than other particles.
With
an instrument that can measure light absorption and attenuation
(the a-c meter), we can start to distinguish living phytoplankton
from dead plankton and other particles without taking water
samples.
|
Optical
backscatter meter (OBS or Light scattering sensor (LSS)
Light
scattering is difficult to measure completely because light
scatters in all directions and the angle of scattering depends
on the particles' sizes. Furthermore, as soon as light bounces
off of one particle it can bounce off of another. We cannot
distinguish whether we are measuring primary, secondary, tertiary,
or greater scattering.
Nevertheless,
small scattering meters have been made that are useful-especially
in turbid water-as they provide a crude measure of the particle
abundance in the water. Scattering meters project a beam of
light into the water while a detector next to the light source
measures the amount of light scattered back into it.
The optical
backscatter meter (OBS) built by Downing Associates and the
light scattering sensor (LSS) built by SeaTech use an infrared
light source and a detector for this purpose. To determine
the amount of particulate matter in the water, these instruments
(like transmissometers) have to be calibrated by filtering
particles from a known volume of water and weighing them very
precisely.
|
Optical backscatter meter (OBS) or Light scattering sensor
(LSS) [11K]
|
Chlorophyll
sensors
Most phytoplankton
produce chlorophyll which they need for photosynthesis. The
amount of chlorophyll in the water should depend on the abundance
of phytoplankton.
This simple
notion is actually more complicated because the amount of
chlorophyll in an organism depends on its species and environmental
factors.
|
| Still,
a measurement of chlorophyll is useful and can be accomplished
with an a-c meter or a fluorometer. The presence of chlorophyll
in seawater causes a dramatic change in attenuation of light
with a wavelength of about 676 nanometers. We quantify that
change with an a-c meter to estimate the amount of chlorophyll
in the water. |
a-c meter [14K]
|
|
Using
a different method, a fluorometer acts as a false sun and
emits a flash of light at one wavelength, which triggers a
response in phytoplankton that causes them to fluoresce, or
give off a tiny amount of light, at another wavelength. The
fluorometer quantifies the light from plankton, and we convert
that to a measurement of chlorophyll in the water. The fluorometer
is calibrated with discrete measurements of known quantities
of chlorophyll.
|
Fluorometer [12K] |
Particle
and Optics Profiling System (POPS)
The Particle
and Optics Profiling System (POPS) is an assembly of instruments
designed to count and measure particles and to determine optical
and environmental properties of water with depth. The heart
of POPS is the Large Aggregate Profiling System (LAPS), designed
and operated by our colleague, Dr. Ian Walsh.
LAPS consists
of a video camera synched with a strobe light which flashes
at predetermined intervals as the system is slowly lowered
through the water. The light is channeled so that it illuminates
only a narrow slab of water. The video camera faces the slab
of light and records images of large particles in the water.
Your eyes
record the same phenomenon when sunlight passes into a darkened
room through a narrow slit in the curtains. You see hundreds
of dust particles wafting through the slab of light, but the
particles on either side of the slab are invisible. If you
put a ruler next to the illuminated particles, you could measure
their size. If you take a photograph of an area of known size,
you can count the number and calculate the size of dust particles
in the air. This is what we do with LAPS in seawater.
The video
images generally show white dots, marine snow, in a dark sea.
They are analyzed with software that counts the number of
particles and determines their maximum and minimum dimensions
if they are not circular. We use these data to estimate a
volume and mass of the imaged particles, which range from
250 microns to several millimeters in size.
In addition
to LAPS, POPS can carry a CTD, transmissometer, fluorometer,
LISST, LSS and a-c meters. All except the CTD are optical
instruments that measure either visibility in seawater or
the sizes of particles.
|
POPS [53K] |
Laser
In-Situ Scattering and Transmissometer (LISST)
A Laser
In-Situ Scattering and Transmissometer from Sequoia Instruments
measures the size distribution of particles five to 250 microns
in diameter. LISST is attached to POPS and generates diagrams
of the size distribution of particles and small aggregates
in the water column without disturbing them by collecting
them in water bottles. Particles' sizes influence their optical
properties and settling dynamics, but many of the aggregates
fall apart in water bottles before they can be returned to
the lab and measured.
To study
smaller discrete particles we collect water samples and precisely
measure the number and size of particles in the one- to thirty-micron
size range with a Coulter counter. The Coulter counter pulls
the water sample through a small hole in a tube through which
an electrical current is also flowing. Particles in the sample
decrease the current flowing through the hole by an amount
proportional to their volume. Doctors use the same type of
instrument to count your red blood cells.
|
Laser in-situ scattering and transmissometer (LISST) [16K] |
Sediment
Trap
Many particles
in seawater are either neutrally buoyant or settle very slowly-only
10 centimeters to 10 meters each day. Most of the optical
signal comes from these small, slowly sinking particles. Other
particles, how-ever, settle quickly-as fast as 10 to 100 meters
each day-and play a major role in biogeochemical processes
in the ocean. To collect the rapidly settling particles we
attach sediment traps to a wire held taut by an anchor at
the bottom and glass floats at the top. The traps are simple
cylinders that collect the settling particles that are carried
into the traps by coastal currents (1-40 cm/sec). In the laboratory
we determine the size and composition of the material the
traps collect.
|
Sediment Trap [7K] |
|
